In its evolution throughout geologic time, the oceanic microplankton became increasingly diverse in major taxonomic representation and biochemical variety, but fluctuated markedly in species diversity, abundance, size range, morphology, and dominant skeletal composition.

Organic-walled phytoplankton of the Precambrian was locally abundant, specimens relatively large, but diversity low. By Ordovician and Silurian times, phytoplankton varied widely in size and morphology; zooplankton included radiolarians and rare tintinnids. Decreasing suddenly in the Late Devonian, both phyto- and zooplankton were rare in the late Paleozoic.

Slow development of plankton in the Triassic left a meager record of acritarchs, coccolithophorids and radiolarians. Rapid diversification characterized the later Mesozoic; abundant dinoflagellates, tintinnids (calpionellids), and planktonic Foraminifera were added during the Jurassic; siliceous phytoplankton arose in the Cretaceous (diatoms, silicoflagellates, chrysomonads), and radiolarians and coccolithophorids expanded.

Another severe reduction in plankton diversity and abundance closed the Mesozoic, hence most groups are poorly represented in the Danian. Paleocene and Eocene diversification and proliferation of phytoplankton (dinoflagellates, coccolithoporids, diatoms, silicoflagellates, ebridians) and zooplankton (radiolarians, foraminifers and some tintinnids) were accompanied by increased morphologic complexity and greater size range. Although plankton abundance, diversity, and complexity fluctuated in the later Cenozoic, no new higher taxa arose.

At present, abundant plankton indicates high productivity, but diversity may be low. Geologically, this abundance was reflected in rapid accumulation of biogenic sediments (chalks, cherts, diatomites); extensive photosynthetic utilization of CO₂ also may have contributed to the accumulation and preservation of calcareous sediments.

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